FEEDING ECOLOGY OF SHARP-SHINNED HAWKS NESTING IN DECIDUOUS AND CONIFEROUS FORESTS IN COLORADO

Feeding ecology of 11 Sharp-skinned Hawk (Accipiter striatus) pairs nesting in aspen (Populus tremuloides), conifer (Abies, Picea spp.), and mixed aspen-conifer habitats in southwest Colorado was investigated during 1988-1989. Small birds (xBAR = 20.9 g, SE = 0.8 g) and mammals (xBAR = 41.1 g, SE = 3.3 g) comprised 91 and 9% of 513 prey identified at nests that fledged at least one young, respectively. Sixty percent of the birds eaten during the hawks' nestling and fledgling stages were nestlings or fledglings. Accordingly, median mass of birds eaten decreased from 17.4 g during incubation to 12.1 g during the nestling stage. Although more birds were consumed than mammals during all nesting stages (birds = 91.1%; mammals = 8.9%), the proportion of birds relative to mammals in diets progressively decreased from incubation through fledging. Taxa of birds in the diet were consumed in proportion to their occurrence in the most abundant of three different habitats surrounding nests, whereas some mammalian taxa were consumed in greater proportion than their relative ''availability'' in these habitats. This suggested that Sharp-shinned Hawks foraged opportunistically for birds, but may have selectively foraged for mammals. Differences in the habits of mammals (e.g., fossorial vs. terrestrial behavior), and hence their relative availability, may explain the apparent selection for certain species of mammals by